The present invention relates to communication with a field sensor, and more particularly to field sensor communication method and system suitable for signal transmission/reception between a plurality of field sensors or communication units
In general, a kind of sensor called a field sensor has a variety of sensors incorporated therein and detects physical quantities inclusive of pressures, flow rates, temperatures and/or the like of various plants to convert them into electrical signals which in turn are transmitted to a receiver instrument through a two-wire transmission line This signal transmission is standardized such that the field sensor delivers an analog current signal in a range of 4 to 20 mA and the receiver instrument or a communication unit receives the analog current signal.
On the other hand, in recent years, a field sensor having a microprocessor incorporated therein has been developed and put to practical use owing to improvement in semiconductor integrated circuit technology. In this type of field sensor, one-directional analog signal communication as well as bi-directional digital signal communication are made on a two-wire transmission line so as to allow remote control of range setting, self-diagnosis and/or the like for the field sensor.
FIG. 9 shows an example of a two-wire field sensor system construction which is well known by, for example, U.S. Pat. No. 4,737,787. In the figure, a field sensor 1 outputs an analog signal, serving as a constant current source which supplies a current corresponding to a detected physical quantity to a signal transmission line 5 by virtue of a voltage supplied from an external power source 4, and a receiver instrument 3 receives the analog signal which in turn is used as a value indicative of the physical value of the field sensor 1. A communication unit 2 is connected between the field sensor 1 and the receiver instrument 3 and external power source 4 and communicates with the field sensor 1 by virtue of a digital signal
As a method of transmitting a signal to a two-wire transmission line is known by, for example, JP-A-59-201535, a method in which a digital signal is superimposed on an analog signal to communicate the digital signal without giving any influence to the value of the analog signal Also, signal transmission with a change-over between an analog signal and a digital signal is known by JP-A-58-48198.
In the above-mentioned prior art, however, no consideration is paid to the case where a plurality of field sensors or communication units are connected to a signal transmission line. Namely, for example, when a plurality of communication units communicate with one field sensor in an asynchronous manner, there arises a problem that signals collide with each other on a signal transmission line, thereby interrupting a consecutive communication operation. Also, in the case where in a period of time from transmission of a signal from a certain communication unit to a field sensor until return of a response signal for the transmitted signal, that is, in an out-of-use time of a signal line, another communication unit transmits another signal to the same field sensor, a disturbance occurs in the operation of communication between the field sensor and the communication units.
As a method of solving these problems may be considered a method in which signal transmission to a plurality of instruments connected to a common line is made in accordance with a predetermined priority But, there is a problem that this method makes the system complicated in construction and high in cost.
Also, since a field sensor is installed in a plant or the like, a power consumption must be suppressed in view of safety. Accordingly, it is not possible to use a medium size or large size processor and it is necessary to simplify a consecutive communication operation.